about projects similar to the project
we were working on, formed a solid
basis for our HVAC design workflow.

We supplemented that “global”
research and workflow by attending
local ASHRAE chapter meetings
and talking with our peers (engineers
and contractors) to understand the
local capabilities and costs of various systems being considered for
application. That basic workflow still
works well today.

Next, when we met with an architect,
and rarely the owner, for the first time,
our workflow went something like this:

1. The architect had previously metwith the owner and determined thetype of building, its site, orientation,structure, and materials and typicallyhad developed a basic floor plan ofthe building based on the owner’sprogram (sometimes that programincluded the type of HVAC systemthat was expected to be designed).

2. Using the research we had
done, we would quickly make some
back-of-napkin calculations of the
expected HVAC loads based on local
rules of thumb and experience. For
example, we often used 400 ft2/ton,

3. Then we would roll out the architect’s drawings and tape a layer of
flimsy vellum over them and begin to
sketch duct layouts and draw boxes
for air-handling units, VAV terminal
units, cooling towers, chillers, boilers, etc., on the vellum.

4. Next, we’d assign some sizes to
the duct and equipment based on
the rules of thumb we were using
for the project and quickly check to
see if the duct and equipment would
fit within the structure and space
allowed by the architect.

5. Finally, we’d take that informationback to the office, make some singleline drawings, develop basic equip-ment schedules, write a narrative ofthe HVAC systems (I hesitate to call ita basis of design [BOD] given today’sstandards), and submit it along withthe other disciplines to the owner andto a local contractor for pricing as aschematic design (SD) package.Things were kept simple, andhigh precision was not demanded.Owners were usually willing to carryreasonable contingencies in theirconstruction budgets, and contrac-tors adjusted their pricing based onsome level of uncertainty in the levelof detail in the SD package. Peoplealso smoked in buildings, and no onehad given much thought to VOCs andtheir impact on IEQ in those days.Energy was considered abundantand inexpensive, so much so thatthe chairman of the regional powercompany where I worked onceproclaimed that nuclear power wasgoing to be so abundant and inex-pensive that there would be no needfor anyone to have a utility meter ontheir building.

Then came the energy crisis of
the late 1970s and early 1980s.
Suddenly, owners, architects, engineers, policy makers, and regulators
became very interested in energy.
Using HVAC energy and economic

I use the terms accuracy, precision,
prediction, and forecast in specific and
calculated ways throughout this article.

As an engineer and building analyst, I
think we often confuse accuracy with
precision when performing BPA and
reporting the results to our clients and
building owners.

In early phase modeling, accuracy ofthe results when comparing alternativesis more important for allowing the ownerand design team to make more informeddecisions than is the precision of theforecasted performance to what may bethe actual performance. A North CarolinaState University website (http://tinyurl.com/hgy2mo3) states:“A good analogy for understandingaccuracy and precision is to imagine abasketball player shooting baskets. If theplayer shoots with accuracy, his aim willalways take the ball close to or into thebasket. If the player shoots with precision,his aim will always take the ball to thesame location, which may or may not beclose to the basket. A good player will beboth accurate and precise by shooting theball the same way each time and eachtime making it in the basket.” If our early phase comparisons useconsistent workflows and methodologiesand trusted simulation tools and provideresults that are “close enough” to knownvalues from prior experience or from publicdatabases of building performance, thenthe results should be accurate enough toinform better decisions without the rigor ofperforming expensive, detailed modelingso early in the process.We do not need to try to forecast pre-cise numbers such as absolute energyconsumption or indoor CO2 levels or worksurface illumination at this stage in thedesign process. Most of the informationwould be too soon obsolete to be useful.I also use the term forecast instead ofthe word predict when referring to esti-mates of future building performance.While the two terms are often seen todayas synonyms, their historical meaningsare quite different. The word predict issomething the town soothsayer might tellyou during a palm reading—a pure guessbacked up by little science.The word forecast was more often usedwhen some form of science or formal ana-lytical process was used to estimate futureperformance, such as the rigorous modelsthe National Weather Service produces toforecast future weather events. A forecastis typically delivered with some statementabout the uncertainty embodied in theresults of the analysis. Since we cannotpredict the future, as engineers we shouldbegin reporting the results of our BPA asforecasts and explain to our clients thepotential range of possible outcomes.Accuracy vs. Precision, Prediction vs. Forecast